Eroding landscapes:
fluvial processes
Feedbacks between
mountain building,
erosion and climate
Mikaël ATTAL
Acknowledgements: Jérôme
Lavé, Peter van der Beek and
other scientists from LGCA
(Grenoble) and CRPG (Nancy)
Marsyandi valley, Himalayas, Nepal
Lecture overview
I. Introduction: mountain building and the critical taper theory.
II. Erosion controls the geometry of mountains?
III. Erosion controls the structure of mountains?
1) Orographic effect.
2) The curious case of the Himalayas.
IV. To which extent does erosion affect deformation in mountains?
“Revisiting river anticlines”.
I. Introduction: mountain building and the critical taper theory.
Convergence
friction
1. Subduction
accretion
Example : Barbados accretionary prism
The Atlantic plate
plunges under the
Caribbean plate
Source: C. Beck, Chambery
Example : Barbados accretionary prism
Structure of the
prism
(seismic imagery +
bathymetry)
Source: C. Beck, Chambery
Convergence
friction
2. Collision
Building of the Alps (schematic representation):
Structural map of the Alps
The “piémontaise” units (dark green) correspond
to the relicts of the Alpine ocean: to the west,
units are European; to the east, they are African
Source: "GEOL-ALP" (http://www.geol-alpes.com), Maurice GIDON, 1998-2003
accretion
Geological cross-section of the Alps (Schmid, 2000)
New tectonic interpretation of the ECORS-CROP profile
NW
SE
Flexural basin
African Units
European Units
Suture
zone
Geological cross-sections across the Himalayas
Indus-Tsangpo suture
is what remains of the ocean
which has been closed due to the
convergence and collision of the
Asia and India plates. To the
south, Indian units; to the north,
Asian units.
Zhao, 1993
Lavé & Avouac, 2001
The critical taper theory
“Mechanics of Fold-and-Thrust Belts and Accretionary Wedges”
Davis, Suppe & Dahlen,
JGR, 1983
Characteristics of the
accretionary wedge:
- basal decollement,
- important compressive
deformation above
decollement, minor
deformation below,
- taper shaped.
Mechanical model
Mechanical model
Coulomb failure
criterion :
  SO  n  pf 
τ = shear traction at failure,
S0 = cohesive strength,
μ = coefficient of friction,
σn = normal traction,
pf = pore fluid pressure.
The critical taper
H
d
gHsin   w gDsin(    )   b    x dz  0
dx O
The Mohr diagram is used to solve the equation and describe the shape of the taper
α
β
where

    f ( , b, )
p f  w gD
 z  w gD
and μ = coefficient of friction
(μb = basal coef.)
  R  F
Linear relationship
between α and β
α
β
Application to natural objects: Taiwan
Topographic
profiles
(Western
Range of
Taiwan):
values of α
α
β
    f ( , b, )
Determination
of the
parameters
producing the
best fit
between
model and
field data
α
β
    f ( , b, )
Other examples:
values of α and β
α
β
    f ( , b, )
Constraining the parameters
α
β
    f ( , b, )
  R  F
Linear
relationship
between α and β
Modification of the equilibrium
Example: mountain building
X
A: subcritical / “stable”  α can increase.
C
B: critical taper. α cannot increase
anymore. If α > critical value, the taper
becomes supercritical / unstable and
collapses.
B
A
C: to carry on growing, the taper cannot
steepen anymore so it has to “expand”
horizontally as well as vertically.
A
B
α
β
C
α
β
α
β
II. Erosion controls the geometry of mountains?
D
Steady-state: FE = FA
C
B
A
A: no topography, FE = 0.
B: mountain grows  FE increases.
C: critical taper stage, slope α cannot
increase anymore.
D: FA = FE  steady-state. The
topography does not evolve anymore.
Flux
D
FA
Willett & Brandon, Geology, 2002
FA = flux of material accreted,
FE = flux of material eroded.
B
A
FE
C
Time
II. Erosion controls the geometry of mountains?
F
D
D-E
Steady-state: FE = FA
A
D: FA = FE  steady-state.
E: drop in FE (e.g., climate change with
less rain)  erosion rate decreases  the
topography is not at steady-state anymore.
F: mountain grows again  FE increases
until a new steady-state is reached (FA = FE)
Flux
D
F
FA
Willett & Brandon, Geology, 2002
FA = flux of material accreted,
FE = flux of material eroded.
B
A
FE
C
E
Time
II. Erosion controls the geometry of mountains?
Steady-state: FE = FA
 Erosion controls the GEOMETRY
of the mountain range
Remark: “real” mountains are more
complex:
- presence of discontinuities (e.g.
faults),
- different lithologies (more resistant in
the core of the range),
- change in crust rheology (e.g. lower
crust partially molten under Tibet  no
basal friction).
Willett & Brandon, Geology, 2002
FA = flux of material accreted,
FE = flux of material eroded.
II. Erosion controls the geometry of mountains?
Evolution of the Alps (Schlunegger et al., 2001, 2002)
Schlunegger et al., 2001
II. Erosion controls the geometry of mountains?
Evolution of the Alps (Schlunegger et al., 2001, 2002)
Schlunegger et al., 2001
Crystalline rocks exhumed
~20 Ma ago  decrease in
erosion rate  range grows
and widens.
II. Erosion controls the geometry of mountains?
F
D
D-E
Steady-state: FE = FA
A
D: FA = FE  steady-state.
E: drop in FE (e.g., more resistant rocks
exposed)  erosion rate decreases  the
topography is not at steady-state anymore.
F: mountain grows again  FE increases
until a new steady-state is reached (FA = FE)
Flux
D
F
FA
Willett & Brandon, Geology, 2002
FA = flux of material accreted,
FE = flux of material eroded.
B
A
FE
C
E
Time
III. Erosion controls the structure of mountains?
1) Orographic effect.
Willett et al., 1993
III. Erosion controls the structure of mountains?
1) Orographic effect.
Dominant wind/rain on retro - side
Willett, JGR, 1999
Dominant wind/rain on pro - side
III. Erosion controls the structure of mountains?
1) Orographic effect.
Field data: Southern Alps, New Zealand
Metamorphism grade
Willett, JGR, 1999
Rainfall
III. Erosion controls the structure of mountains?
1) Orographic effect.
Field data: Olympic Mts, NW USA
Metamorphism grade
Laumontite (L), prehnite-pumpellyite (Pr+Pu),
pumpellyite (Pu), and chlorite-epidote (Cl+Ep).
Willett, JGR, 1999
Rainfall
III. Erosion controls the structure of mountains?
1) Orographic effect.
Southern Alps, New Zealand:
erosion on the retro side
Willett, JGR, 1999
III. Erosion controls the structure of mountains?
1) Orographic effect.
Olympic Mts, NW USA:
erosion on the pro side
Willett, JGR, 1999
III. Erosion controls the structure of mountains?
2) The curious case of the Himalayas
Break
NOTE: deadlines for essays =
Monday 23rd March, 16:00
(Patriat & Achache, 81)
III. Erosion controls the structure of mountains?
2) The curious case of the Himalayas
III. Erosion controls the structure of mountains?
2) The curious case of the Himalayas
India – Asia collision
mm/yr
(Patriat & Achache, 81)
~50 mm/yr at
the moment
Convergence rate India / Asia
(Tapponnier et al., 2001)
 The highest mountain range + the largest high-altitude
plateau on Earth